Polar modulation is a technique where a signal, or carrier, having constant radian frequency ω, is time-varied in both magnitude and phase. Polar modulation transmitters transmit information that both the magnitude (R) and the phase (θ) of a signal carry simultaneously. There are many benefits to using polar modulation to transmit information, particularly for wireless handset transmitter designs. Polar transmitters may receive baseband signals represented in Cartesian form as an in-phase (I) component and a quadrature (Q) component. An IQ baseband signal may be converted to polar form in terms of its magnitude R and phase θ signals. The magnitude R is referred to as the amplitude component, or amplitude signal, and the phase θ is referred to as the phase component, or phase signal. A coordinate rotation digital computer (CORDIC) algorithm may be employed to convert the IQ baseband signals to polar form amplitude R and phase θ signals. The amplitude R and phase θ signals may be processed in separate amplitude and phase paths and may be recombined at the output of the power amplifier. The IQ components may be reconstructed by additional processing downstream of the power amplifier output.
Polar modulation techniques allow a nonlinear device, such as a power amplifier, to operate in the saturation (nonlinear) region with higher power efficiency and longer battery lifetime. As wireless phone standards evolve from 2G to 3G and beyond, for example, EDGE (Enhanced Data GSM Environment) and UMTS (Universal Mobile Telecommunications System), the demand for non-constant envelope modulation using a polar transmitter is growing rapidly. This is due in part to the potential for benefits in terms of hardware, power savings, and multi-mode flexibility. Nonlinear devices may be used for this type of polar transmission.
In nonlinear devices, waveform quality typically increases with a more linear output response. However, some nonlinear devices operate more efficiently when the output response is nonlinear—for example, when a power amplifier approaches saturation. As a result, there is often a tradeoff between waveform quality and efficiency. For example, when a nonlinear device approaches saturation or starts to exhibit nonlinear qualities (which may improve efficiency), the waveform quality may be degraded and may not meet the specific requirements and standards. Alternatively, if nonlinear devices are set to operate in linear regions to meet quality standards or requirements, then power consumption and current drain may be degraded because the device is operating at a lower efficiency level.
Correction of the nonlinearity of the power amplifier becomes extremely challenging as the polar modulation technique is applied to non-constant envelope modulation. Two primary approaches exist: open-loop correction and closed-loop error-based correction. Open-loop correction, which typically involves a lookup table (LUT) is relatively simple, but needs significant manufacturing calibration for collecting tables or calculating the coefficients, and suffers performance loss if the device nonlinearity varies once out of the manufacturing environment and the pre-collected tables or pre-calculated coefficients are no longer accurate enough. On the other hand, error-based closed-loop correction, either adaptive or non-adaptive, may fail to deliver accurate correction to severe nonlinearities, especially for a high gain loop with large delay. Thus, there is a need for a reliable and efficient digital polar transmitter.